1. Field of the Invention
This invention relates to golf clubs, and more particularly to a wood or iron having a shaft whose flexibility can be incrementally increased or decreased while playing or practicing.
2. Description of the Related Art
A golf swing is a set of highly complex body movements requiring precise coordination of the hands, arms, shoulders, torso, hips, legs and knees, occurring in proper sequence. As the body coils and then uncoils, power is transferred from the body through the arms and wrists to the club grip, along the shaft, into the clubhead, and into the ball.
In a full range of motion swing, the position of the leading arm (i.e., the left arm of a right-handed golfer, or the right arm of a left-handed golfer) is critically important. The backswing is performed as a unitary motion, the entire front side of the body moving together as the knees, hips, trunk and shoulders are rotated, with the leading arm pushing the trailing arm back, and the leading elbow and arm remaining straight. To achieve the broadest possible swing arc, the club is swung straight back from the ball, without breaking the wrists, for as long as the turning of the shoulders and hips will allow. The golfer tries to keep the clubface square to the target line for as long as possible without turning the hands and wrists under, and also tries to keep the leading arm and club in a straight line until the momentum of the swinging clubhead causes the wrists to begin cocking naturally as they reach about hip height. The angle between the hands and shaft is maintained, with the trailing arm bending at the elbow and the leading arm remaining straight. The one-piece motion forces the shoulders to turn from the very beginning of the backswing and ensures they will go on turning until the top of the swing is reached. As the arms stretch and turn, the hips are also forced to turn. At the top of the backswing, the shoulders have turned about twice as far as the hips, the leading arm is straight, and the forearms have rotated. The leading forearm is pronated, i.e., rotated clockwise for a right-handed golfer or rotated counterclockwise for a left-handed golfer, and the trailing forearm is supinated, i.e., rotated counterclockwise for a right-handed golfer or rotated clockwise for a left-handed golfer, causing cocking of the wrists with the shaft generally perpendicular to the leading forearm. Ideally, the back of the leading hand, the wrist and the forearm are in a straight line, with the shaft parallel to the ground and to the target line, and the clubhead pointing toward the target.
As the arms reach their fullest extension and the weight of the clubhead causes the wrists to attain their maximum cocking but before the hands reach their highest level or the shoulders finish turning, the lower torso, hips, legs and feet already have initiated the downswing. The golfer pushes hard off the inside of the back foot, throwing weight to the inside of the front foot. The front knee is pulled laterally toward the target and is well forward of the ball before the hands have descended even a few inches. The wrists are kept cocked and the head is kept back behind the ball. The club is pulled into action by the uncoiling of the body and leading arm. Because of the movement of the lower body toward the target and the delay in uncocking the wrists, the arc traversed by the clubhead on the downswing is steeper than the arc on which the clubhead was taken back.
In the hitting zone the hips, which were moving laterally in the same forward direction as the knees, begin to turn with respect to the target line. By turning, the hips "clear" a path for the arms to swing past the body. The thrusting legs and hips, by forcing the shoulders to turn, accelerate the arms and club. Just before impact, a point is reached where no further acceleration is possible and, because of centrifugal force, the club must be released into the ball. At that point, the wrists are forced to uncock spontaneously.
As the wrists uncock and the back arm starts to straighten, the fully released clubhead whips toward the ball. At impact, the back of the leading hand faces the target, and the leading arm and shaft form a straight line so that the leading hand and arm are slightly ahead of the ball. As the clubhead swings through the ball toward the target, the leading arm is kept straight and moves directly toward the target. The golfer must avoid any independent turning or twisting of the club with the hands and wrists.
The release phase is entered just after impact. The trailing arm straightens, but there is no breaking down of the leading arm at the wrist or elbow. Nor is there independent rolling, turning or twisting of the hands, wrists or forearms, until the momentum of the club, combined with the turning body, forces the body to turn and swing to the opposite side of the target line. The leading arm is supinated and the trailing arm is pronated, the forearms being opposite to their rotational position at the top of the backswing.
As millions of golfers can attest, it is difficult to meld the separate motions comprising a full range of motion swing so as to achieve impact with the clubface generally square to the ball while the clubhead is moving at a high rate of speed. The present invention entails increasing clubhead speed by flexing the shaft to impart to potential energy additional to that accumulated during the backswing, which is converted into extra clubhead kinetic energy in the hitting zone and at impact.
Golf clubs having shafts with modifiable flexure are disclosed in the related art. U.S. Pat. No. 2,992,828 to W. A. Stewart discloses a club having a hollow shaft which is prestressed so as to remain relatively straight during the backswing and downswing, compared to a conventional shaft. In one embodiment a wire inside the shaft and adjacent to the leading edge is maintained under tension between plugs at the top and bottom ends of the shaft. The upper plug receives a bolt which when rotated causes the plug to act as a nut and travel upwardly on the bolt, increasing tension in the wire and thereby compressing the shaft leading edge. Tightening the wire tends to bow the shalt in a direction opposite to the direction the shaft would normally bend on the downswing. The wire is not tensioned sufficiently to bow the shaft but just enough to prestress the shaft or apply a bending stress in a direction opposite to the bending stress applied to the shaft during the downswing. In another embodiment an elongated column member extending the length of the shaft between upper and lower plugs has its trailing edge compressed by screwing a bolt down through the upper plug. This puts tension on the trailing side of the shaft, inducing a bending stress in the shaft opposite to the downswing bending stress.
U.S. Pat No. 4,685,682 to J. T. Isabell discloses a golf club connected to a mechanism which increases or decreases shaft flexibility by varying the tension in a wire extending between the handle and clubhead. The wire is offset from the shaft by a bridge which maximizes the effect of small changes in wire tension provided by movement of a threaded connection between the wire and clubhead. Increasing the tension in the mechanism decreases shaft flexibility; decreasing the tension increases flexibility. The device is used to determine the degree of shaft flexibility which maximizes an individual golfer's clubhead speed at impact, so that a set of clubs can be made that have the same flex characteristics.
U.S. Pat. No. 5,865,688 to S. W. Bae discloses a golf club having three flex points along its shaft. At each point, the shaft diameter expands to permit the shaft to flex at that point. When the club is swung, the shaft flexes from a high flex point (i.e., a location proximate to the handle) to a mid flex point to a low flex point (i.e., a location proximate to the clubhead).
U.S. Pat. No. 5,931,744 to L. E. Hackman discloses a method for reducing the stiffness of a hollow golf club shaft divided into three segments with each pair of segments connected by cone-shaped bands. The stiffness is changed by slicing or abrading the inner surface of the sidewall which typically is made from a flexible matrix material such as an epoxy, in which elongated, high strength graphite fibers are embedded. Most of the fibers are arranged longitudinally to provide strength and stiffness to resist bending of the shaft. Severing or removing selected fibers reduces the longitudinal stiffness.
None of these references address the problem of providing a golf club whose shaft easily can be made more or less flexible so that a golfer can determine through experimental trial which degree of flexibility best suits his particular full range of motion swing.